Field of the Invention
The invention relates to a stacked filter and more particularly to a stacked filter for an image sensor having infrared (IR) pixels integrated with red (R), green (G) and blue (B) pixels together in a single sensor for time-of-flight (ToF) technology application.
Description of the Related Art
Currently, time-of-flight (ToF) technology has been used extensively in modern industries to provide three-dimensional (3D) imaging by using a low-cost complementary metal-oxide semiconductor (CMOS) pixel array together with a modulated light source. 3D ToF cameras are used for many different applications, for example, profile inspection of manufactured goods, computer-aided design (CAD) verification, geographic surveying and object imaging.
A 3D ToF camera works by illuminating the scene with a modulated light source, and observing the reflected light. The phase shift between the illumination and the reflection is measured and translated into distance. Typically, the illumination is from a solid-state laser or a light-emitting diode (LED) operating in the near-infrared range of about 850 nm, invisible to the human eye. An image sensor designed to respond to the same spectrum of the modulated light source receives the light and converts the photonic energy to electrical current to obtain distance (depth) information of the scene.
In general, a filter is disposed in front of the image sensor to obtain a better signal-to-noise ratio (SNR) of the distance (depth) information. The conventional filter is usually fabricated by a multi-film interference technology. When a large inclined incident light falls on the conventional filter, a blue-shifting always occurs at the conventional filter and causes the spectrum of the conventional filter toward a lower wavelength band. Therefore, the image sensors containing the conventional filter require a telecentric lens with 0° or small chief ray angle (CRA) to overcome the blue-shifting caused by large inclined incident light.